Blast furnace



Aug. 6, 1935. i FRNKL 2,010,073

BLAST FURNACE Filed Oct. 14, 1931 6% v 20,?? o M@ Patented Aug. 6, 1935 UNITED STATES PATENT OFFICE BLAST FUBNACE Application October ln Germany The invention relates to blast furnaces and to a construction thereof whereby said furnaces may be operated with a blast containing a percentage of oxygen greater than that in air.

The use of a blast enriched in oxygen in the operation of blast furnaces for the production of iron is attended by the difllculty that every kind of furnace lining, especially within the smelting zone, is soon Worn by the slag, by melting and by mechanical abrasion, down to where the water cooled mantle stops further damage.

Cooling, however, involves great losses of heat in the very part of the furnace where the raising of the necessary high temperature is particularly costly. This loss of heat has to be compensated for by the addition of oxygen to the blast and by heated air.

The difiiculty may somewhat be lessened by the use of carbon as the furnace lining, as this is not destroyed by the slag and does not melt off. However, as carbon offers little resistance to mechanical stress or to the attacks of oxygen and oxides, as carbon dioxide and iron oxide, it is not possible to ensure durability of the lining Without taking certain particular steps.

In blast furnaces operated in the hitherto usual manner, during combustion a considerably higher temperature develops along the outer zone than within the center of the hearth. Combustion takes places only in the peripheral zone, the gases developed pass toward the center and at the same time toward the top, in consequence of the combustion taking place in this manner a cone shaped core of fuel remains in the center of the hearth. This core takes no part in the combustion, since the fuel at or near the tuyres burns only to CO, which does not act on the carbon.

The conditions under which this combustion takes place are highly deleterious in their eiect on the furnace lining because, rst, the lining is exposed to the highest temperature, second, the charge mainly descends along the lining, and third, there is always the danger that the carbon dioxide will act on the lining (in case carbon bricks are used to prevent melting), since combustion takes place near the lining.

The unburned fuel cone causes an irregular descent of the charge and consequently many interruptions in the furnace operation, as it is obviously impossible for the charge to descend evenly and in its entirety if it rests continually on a compact cone shaped core of fuel and slag which takes no part in the combustion.

It follows that the charge must slide down along 14, 1931, Serial No. 568,7

November 18, 1930 5 Claims. (CL 2GB-25) the outer surface of'this cone. being supported in the middle, and will descend in the center of the stack more slowly than in the vertical layers between the side of the furnace and the center core.

As a consequence the ore will only be reduced satisfactorily at the middle of the stack, the faster descending part of the ore being subjected to the reducing influence of the reducing gases for too short a time. Further, a sticking of the charge in the stack also results, since the charge instead of descending as a whole, slides within itself, thereby increasing friction and the resistance to flow.

The operation of a furnace with a blast strongly enriched in oxygen in the hitherto usual manner, that is by peripheral combustion, is quite impossible, as then the disadvantages above mentioned would especially pertain, inasmuch as the combustion would penetrate less deeply than 2 when operating with hot air.

A blast containing about 40 to 45 per cent Oz in consequence of its reduced nitrogen content burns carbon to CO so readily that the process requires much less space than when using hot air containing 21 per cent O2. Central combustion is in this case still more important than in the case of hot air. l

It is an object of the invention to provide a furnace in which an ore charge may be subjected to a blast enriched in oxygen without the attending diiilculties hereinbefore mentioned.

It is a further object of the invention to provide a reducing and smelting furnace in which the formation of a self-renewing. protective and isolating layer shall take place whereby the stack lining is protected from deterioration.

Another object of the invention is to provide a furnace in which combustion of the aforesaid protective layer and of the stack lining are avoided and at the same time the formation of a non-combusting fuel cone in the center of the hearth is prevented by causing combustion to take place in the center of the hearth and away from the outer or peripheral zone.

A further object of the invention is to provide a furnace in which the stack lining is protected against the detrimental influences of the combustion gases which are at a high temperature and pass upwardly in a slanting direction toward the periphery of the hearth and through the protective layer, by admixing another less highly heated thermal control gas with the hearth gases above the smelting zone; wherein the quantity of combustion gases from the hearth is reduced by an amount equal to the amount of thermal control gas added above the smelting zone by separating a corresponding quantity of nitrogen from the air blast introduced into the hearth or below the smelting zone, and in which reduction ofcarbon dioxide to carbon monoxide in the reducing stack will in a large measure be avoided without dispensingl with the charging of coke into the furnace stack.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the apparatus embodying features of construction, combinations of elements and arrangement of parts all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention reference should be had to the following detailed description taken in connection with the accompanying drawing, in which:

Fig. l represents a blast furnace having a widened hearth section and charged with a column of intermixed coke and iron ore, and

Fig. 2 illustrates a blast furnace having a widened hearth, the furnace being charged with alternating columns of iron ore and flux and of coke and being connected with heating chambers.

According to the present invention the first step taken to ensure durability consists in providing a self-renewing, stagnating protective and isolating layer composed of fuel and slag which in a manner hereafter to be 'lescribed is prevented from burning and does not take part in the downward movement of the charge.

The formation of a self-renewing protective layer in accordance with the invention is however attended by a certain disadvantage which is counteraoted by the constructions herein de-A scribed.

The portion of the protective layer situated above the zone of combustion will consist ofcoke only, as at the prevailing high temperature at this point all meltable material will melt out of the coke. This layer consequently offers less resistance to the combustion gases which pass from the widened part 0f the hearth, a, into the stack, than does the ore-mingled charge, b. The gases will therefore pass into the stack through the protective layer.

Since no appreciable amount of heat need be used within the protective layer for smelting, the gases having a temperature of about 2000 C. on passing from the widened part of the hearth, a, into the stack Would strike the lining of the stack and cause its destruction, if nothing were done to prevent it.

In order to avoid damage to the stack lining and to supply to the stack an additional amount of heat, air, an oxidizing, indifferent or reducing gas, at a temperature of about 800 C. is introduced by means of gas inlets k and k" into the stack at the point of the peripheral zone Where the hearth adjoins the stack. The introduced gas mixes within an annular canal, p, with the much hotter gas emanating from the hearth, thereby lowering the temperature of the hearth gas to a point which decreases the danger to the shaft lining. The introduction of gas into the annular canal, p, causes the pressure within it to rise above atmospheric, which prevents the rise through the porous protective layer of undesirably large quantities of combustion gas.

The gas blown into the annular canal at about 800 C. serves a double purpose, nrst, it saves the stack lining from destruction and, second, it replaces heat which is lost when operating with a blast enriched with 40 to 45 per centmxygen, and which otherwise would have to be supplied by heated air.

But since, if greater flue gas heat losses are to be avoided, only a certain quantity of gas, proportionate to the weight of the charge, m'ay be sent through to supply the heat needed in the stack, the condition on which additional gas may be blown in is that the quantity of hearth gas is lessened by an amount equal to that of the thermal control gas-"Pto be blown in above the smelting zone, which condition is provided by separation of nitrogen from the blast.

The objects of the invention may be accomplished by sharply enlarging the interior of the furnace near point p, where the stack adjoins the hearth, and near point j, below the zone of combustion, narrowing it again.

The widened and narrowed portions are furnished with carbon bricks; behind these the lining may consist of the cheaper ordinary lining bricks. By the abrupt widening of the hearth where it adjoins the stack the formation of a protective layer consisting of fuel and slag is made possible, which continually renews itself.

It is necessary'also to preserve the protective layer composed of fuel and slag from burning before it glides down, because otherwise the furnace lining would be destroyed by mechanical wear.

To this end the blast nozzles, h, through which 42 to 45 per cent oxygen is introduced, are so arranged that they project into the combustion space, r, to a distance such that combustion (to CO) takes place Within the central portion. It is known that when carbon is burned to CO, the zone of combustion is relatively small, inasmuch as combustion takes place only in front of the blast nozzles. The CO generated is, unlike CO2, unable on its passage through the smelting zone to react with carbon, consequently the protective layer of the furnace lining is not destroyed. In the center of the combustion space, due to the combustion of coke, a small hollow will periodically form which caves in as soon as it has attained a size such that the weight of the column, b, of the charge will not be supported. If at times the hollow becomes so large, that the protective layer also is affected, no disadvantage arises because after the collapse of the hollow the protective layer will soon be renewed.

It is only necessary to keep the diameter of the space enclosed by the protective layer of such size, that regular periodic extensions of it into the protective layer are avoided. An unburned core of fuel, remaining within the center of the space and acting as a prop should be avoided.

Further, the hot gases of combustion are so guided that they effect a melting of the mixture of ore and flux. These gases tend to follow the line of least resistance, and consequently pass upwardly in a slanting direction through the porous protective layer toward the periphery of the hearth. They accordingly would not penetrate the mixture of ore and flux, which would result in a lowering of the output and probable damage to the stack lining. That part of the highly heated gas rising from the hearth and passing through the protective layer will within the annular canal, p, formed at the lower end of the stack by a sharply angular enlargement of the hearth be mixed with a less highly heated reducing gas which is blown into the canal through the gas inlets k' and k".

'I'he furnace therefore is considerably widened at the hearth and the walls of the widened hearth disposed in angular relation, preferably forming a right angle, to the walls of the stack, as shown in the drawing, not only to occasion the formation of a layer protecting the lining, but also in order that the slope of the charge may form the annular canal, p, whereby the gas to be introduced is evenly dmixed and distributed about the stack.

The operation of a furnace is accordingly effected by forming a protective and isolating layer consisting of fuel and a mixture of ore and flux due to the sharply angular shape of a widening at the lower end of the stack, whereby the material composing the protective layer is without the combustion zone in the center of the hearth; by occasionng the combustion near the center of the hearth, in order to prevent formation of an unburned fuel cone which would hinder the even descent of the charge in its entirety; by admixing a less highly heated gas within an annular canal, p, with that portion of the hot gases which passes in a slanting upward direction through the porous protective layer toward the periphery of the hearth and into the stack, in order to lower the temperature of said hot gases to a point at which they will not endanger the stack lining, and also to compensate for the decrease in the heat of the blast when oxygen is used; and by reducing the quantity of gas passing from the hearth into the stack, by separating a large quantity of nitrogen from the blast and, subsequently, in substitution thereof, blowing in above the smelting zone an additional heated gas.

The foregoing procedures are important in the operation of a blast furnace with a blast strongly enriched in oxygen, because if a large quantity of nitrogen which ordinarily acts as a ballast is v separated from the blast, then the blast carries into Fe only at a temperature of upward of 850 C.

At this temperature carbon dioxide, which is formed when CO reacts with ore oxygen, will, in

y the presence of carbon be reduced to CO-at considerable expenditure of heat, inasmuch as the reaction is distinctly endothermic.

On the other hand, indirect reduction of the ore even at temperatures above 1100 C. is unfavourably infiuenced by the fact that the ore will sinter and thus become inaccessible to indirect reduction by CO. The indirect reduction of FeO tol Fe to any extent is thus possible only at temperatures between 850 C. and 1100 C., that is within relatively narrow limits.

' 'I'his reduction consumes a little more heat than is expended in reacting CO with FeO, namely 150 cal. per kg. of Fe. Since this heat in the reducing stack of the furnace may be obtained only from the sensible heat of the gases, and since per kg. of iron produced not much more than 4 kg. of gas circulate in the stack, every kg. of gas is deprived of a quantity of sensible heat amounting to The reduction of carbon dioxide (formed during reduction of FeO with CO) would consume additional heat. The amount of this heat is a little more than 1600 cal. per cubic meter of reduced carbon dioxide. Since during the production of The reducing process is confined to a 'range of 250 C. (that is the range between 850 C. and 1100 0.), and hence the charge must be heated to a temperature of between 850 C. and 1100 C. which alone absorbs four fifths of the sensible heat of the gases.

The presence of solid carbon consequently markedly impedes the indirect reduction of FeO, due to the formation of carbon dioxide; on the other hand charging coke into the stack cannot be avoided without incurring other disadvantages.

The charging of the furnace to effect this result is carried out in a manner such that every ore charge is not followed by a coke charge (as in Fig. l), but rather the stack is filled to about one half its height with ore and limestone flux, b", only, and the other half with coke, b', only (as in Fig. 2), so that the carbon dioxide generated during the ore reduction will have no opportunity for conversion to CO by contacting with glowing coke in the ore column.

The absence of coke in the ore column however has the disadvantage that the reducing gases penetrate it only with difficulty, because the ore shortly before melting will, without the loosening effect of the coke charge, sinter and thus form an almost impenetrable plastic mass. In a furnace of ordinary construction for the normal manner of operation, this process cannot be carried out, because a plastic mass will be formed at a temperature above 1100 C. at which indirect reduction of FeO to Fe takes place, or rather, which is somewhat higher than needed.

The procedure herein described effectively prevents sintering of the ore column until it is within the hearth-which is consider-ably wider than the reducing and charging stack-in that the gas which streams from the hearth into the stack, and which is much too hot, will be cooled, so that sintering cannot occur, by mixing it with an additional gas, which is blown into the annular canal, p, above the smelting zone at a temperature of about 800 C.

But while in the first case, in consequence of the loosening effect of the coke in the ore charge, the gas may pass into the stack mainly through the charge and only partly into the annular canal, p, in the second case all the gas is, intentionally, directed around the plastic mass into the annular canal, p, because, in the absence of coke, it could not penetrate the plastic mass. It is above this plastic mass, and only after the temperature of the gas has been lowered to about 1100 C. by mixing it with a less highly heated gas, that the combustion gases enter into the ore column which lpossesses sufficient penetrability because sintering has been prevented.

Iri this secondv case, smelting off of the ore charge takes place onlyalong the outer surface of the plastic mass, as the gas from the center of the hearth passes in a slanting upward direction towards the periphery thereof, and into the annular canal, p, that is without penetrating the ore charge, by peripheral circulation.

This mode ofcarrying out the process is provided for in the apparatus represented in Fig. 2. It differs from that shown in Fig. 1 only by dividing the charging of ore and coke into a high ore column, b and a high coke column b. It dii'- fers as to the course of the smelting and reducing however, in that the gases are, in their entirety conducted aroundthe plastic coke free mass of vthe ore column, and only after cooling to about 1100 C. by means of an added thermal control gas do they penetrate the ore column above the plastic mass.

Fig. 2 also shows the plant which heats the gas to be added to about 800 C. This plant consists of two gas heaters c' and c", which alternately and periodically are hot blasted by gas firing.

Since certain changes'in carrying out the above constructions set forth, which embody the invention may be made without departing from its scope, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

It is also understood that the following claims are intended to cover all of the generic and specific features of the invention herein described,

and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Having described my invention, what I claim as new and desire to secure by Letters Patent is:

1. A blast furnace comprising incombination a stack, a hearth at the base and wider than said aoioms stack, blast inlets near the base of said hearth and gas inlets st the Junction of said stack and said hearth.

2. A blast furnace comprising a stack of downwardly increasing cross sectional area, an annular flange about the base of said stack, a hearth positioned beneath and having a cross sectional area greater than said stack and in immediate connection therewith, blast inlets near the base of said hearth and gas inlets positioned at said annular stack ange.

3. A blast -furnace comprising in combination a stack, a hearth joined to the base of said stack, said hearth being wider than said stack and the walls of the hearth adjoining the stack being disposed substantially at a right angle to the walls of the stackwhereby an annular canal is formed when said furnace is charged and operating and the lower portion of said hearth being narrowed, blast inlets near the base of said hearth and gas inlets at the junction of said stack and said hearth.

4. A blast furnace comprising in combination a stack, a hearth positioned at the base of said stack, said hearth being wider than said stack and the walls of the hearth adjoining the stack being disposed substantially at a right angle to the walls of said stack whereby an annular canal is formed when said furnace is charged and operating blast inlets near the base of said hearth and gas inlets at the junction of said stack and said hearth.

5. A blast furnace comprising in combination a stack, an annular ilange about the base of said stack, a hearth positioned beneathand having a cross-sectional area greater than said stack and in immediate connection therewith, blast inlets near the base of said hearth and gas inlets at the junction of said stack and said hearth. 

